1. Field of the Invention
This invention relates to a hydraulic pressure control system for a hydraulically operated vehicle transmission, more particularly to a hydraulic pressure control system for a hydraulically operated vehicle automatic transmission which discriminates or estimates whether frictional engaging elements such as clutches are normal, and controls the supply of hydraulic pressure to the frictional engaging elements based on the result of the discrimination, thereby obtaining optimum gearshift characteristics.
2. Description of the Prior Art
The hydraulically operated vehicle automatic transmission has multiple clutches, brakes and other such frictional engaging elements which are engaged by the hydraulic pressure (oil) (Automatic Transmission Fluid). The transmission performs gear shifting, i.e., switches between frictional engaging elements, by releasing (disengaging) one element and engaging another. For preventing unexpected shock during gear shifting, it is necessary to control the co-engagement amount of the two (releasing and engaging) frictional engaging elements.
The co-engagement amount of the elements is, however, extremely difficult to optimize. A particular problem is that when the co-engagement amount is too small, the engine suddenly revs and produces a large gearshift shock. The practice adopted to avoid this has been to prevent revving of the engine during gear shifting by setting the co-engagement amount to a relatively high level at which the total amount of torque transferred by the engaging and releasing elements is greater than the amount of torque input to the transmission.
The slight interlock state this produces in the transmission causes a drop in the transmission output shaft torque. Since this feels like an engine braking effect to the driver, particularly when the drop is amplified by change in the oil temperature or by the aging of the transmission elements, it degrades drivability.
One example of a technology for accurately controlling clutch hydraulic pressure is disclosed in Japanese Laid-Open Patent Application No. Sho 60(1985)-231,056, which is provided with linear solenoid valves, duty solenoid valves or other such solenoid valves for controlling hydraulic clutch pressure. More specifically, it controls the hydraulic pressure applied to the clutches through solenoid valves so as to regulate the rotational speed of a member, such as the transmission input shaft, whose rotational speed changes during gear shifting to follow a desired change rate.
When this technology is applied, for example, to conduct clutch hydraulic pressure control for lowering the co-engagement amount of the clutches so as to temporarily reduce the transmission input torque to a level at which the total amount of the torques transferred by the engaging and releasing clutches does not produce a large drop in the output torque or revving of the engine, it becomes important to discriminate or estimate whether or not the clutches are normal, specifically, for example, whether or not the hydraulic pressures applied thereto are excessively lower or higher than desired, more specifically, whether or not the clutches are good (condition) in torque transmission amount or capacity and suitable for the intended control. The prior art mentioned above gives no consideration to this point.
An object of this invention is therefore to overcome the aforesaid shortcoming of the prior art by providing a hydraulic pressure control system for a hydraulically operated vehicle transmission which discriminates or estimates in a simple manner whether or not frictional engaging elements are normal, and based on the result, conducts hydraulic pressure control to prevent occurrence of gearshift shock without engine revving, thereby achieving good gearshift characteristics and enhancing drivability.